The present invention relates to a power supply circuit for driving a liquid crystal display device.
As a conventional power supply circuit for driving a liquid crystal display device, a power supply circuit using a switching regulator as shown in a circuit diagram of FIG. 3 is known. More specifically, a switching regulator and a transformer are used to derive positive and negative power supplies necessary for a drive voltage.
In a liquid crystal display device using a driving method such as multiline addressing (MLA), as shown in FIG. 2, voltage relationships of VH, VL, and V1, which are drive voltages on a common side, must satisfy the equality VHxe2x88x92V1=V1xe2x88x92VL. If this equality is not satisfied, tailing and the like are caused, resulting in damage to the display quality. In a conventional power supply circuit, as shown in FIG. 3; a transformer generates positive and negative voltages. However, since it is hard to obtain a precise winding ratio of the transformer in the conventional power supply circuit, there arises the problem that the above equality is difficult to be satisfied.
A circuit shown in FIG. 4 has been proposed to solve this problem. A drive voltage on a segment side is created from the VH voltage by resistance dividing. Since the resistance value of a resistor used for dividing the voltage is high at this time, in case that the voltage is actually supplied to a liquid crystal panel, a buffer circuit having an amplification factor of 1 and using an operational amplifier is required between the dividing resistance and the liquid crystal panel. A voltage higher than the output voltage of the operational amplifier must be applied to a positive power supply of the operational amplifier. Here, the difference between the positive power supply voltage and the output voltage of the operational amplifier is the loss of the power supply circuit. If the difference in the voltage is large, the loss is accordingly large, which makes it difficult to implement a liquid crystal display device with low power consumption.
Further, there is also the following problem. That is, in a liquid crystal panel, both the common voltage and the segment voltage have to be varied according to the number of scanning lines. Therefore, the output voltage of the operational amplifier forming the buffer circuit varies according to the duty of the liquid crystal panel, and thus, in order to implement a liquid crystal display device with low power consumption, there is a problem in that the positive power supply voltage has to be changed according to the size of the panel.
In order to solve the above problems, according to the present invention, a power supply circuit for liquid crystal for dividing and inputting to a buffer circuit a voltage on a common side to be applied to a liquid crystal panel to generate a bias voltage on a segment side is formed such that the divided voltage on the common side is used as a reference voltage for a switching regulator, and an output voltage of the switching regulator is used as a positive power supply voltage of the buffer circuit.